In prior art, the structure of one useful form of semiconductor lasers falls into the category of vertical cavity (or simply "vertical") lasers. In a vertical laser, there is an active region which includes a planar pn junction. Typically the plane of this pn junction is parallel to a major surface of a semiconductor substrate body, the major surface of the substrate being considered arbitrarily to be horizontal. In a vertical laser, light is emitted from the top or the bottom (major) surface, or both, of the semiconductor body, a vertical optical cavity being created therein by virtue of semi-reflecting optical mirror(s) located on the top or bottom surface thereof, or both.
The structure of a vertical laser can be made circularly symmetric. Therefore, a vertical laser can have the advantage of relatively low astigmatism as compared with other lasers, such as "edge" lasers in which light is emitted from a side edge of the semiconductor body.
Typically, in a vertical laser each mirror(s) is formed by a quarter-wavelength stack, such as a stack formed by two semiconductors with differing refractive indices, which thus form a semiconductor superlattice. A vertical laser can be built as a double heterostructure, for example, by succesive epitaxial growth of the following semiconductor layers in spatial sequence upon a semiconductor substrate: the bottom mirror, a bottom optical cladding region, the active region, a top cladding region and the top mirror.
In an optically pumped semiconductor laser, optical radiation of wavelength(s) shorter than that (those) to be emitted by the laser is directed upon the laser to create an electronic population inversion. In an electrically pumped vertical cavity semiconductor laser, top electrodes are formed on the top major surface of the top mirror and on the bottom major surface of the substrate, for electrical pumping. Many such vertical lasers can be built on a single such substrate, as by trench or other isolation, in such a way that the intensity of light--e.g., ON vs. OFF-emitted by each laser can be controlled by an electrical signal independently of all other lasers on the substrate. Thus, vertical lasers appear especially attractive for use in practical applications where more than one independently controllable source of light is desired. Typically the amount of light emitted by each such vertical laser is determined by the electrical current injected into the laser through the top electrode. Alternatively, many separate vertical lasers can be mass produced from the single substrate, as by masking and etching apart the individual lasers.
In prior art, the semiconductor substrates that have been used have been confined mostly to gallium arsenide or indium phosphide. Such lasers typically entail very near lattice-matching requirements of the double heterostructure, in order to achieve the high quality (low defect density) epitaxial growth needed for the desirably low optical absorption and high quantum efficieny of light emission.
In the case of gallium arsenide (GaAs) based lasers, the required lattice-matching can be achieved with the ternary semiconductor aluminum gallium arsenide (Al.sub.x Ga.sub.1-x As with x anywhere from zero to unity), because of the special property of sufficiently close lattice-matching between the binary semiconductor GaAs and the ternary semiconductor Al.sub.x Ga.sub.1-x As for any x between zero and unity. On the other hand, in order to achieve reasonably large optical reflectivity at each interface between contiguous layers in the mirror stack, the refractive indices and hence the chemical compositions of the two layers should be reasonably disparate, in order to avoid the need for unduly large numbers of layers in the mirror stack--i.e., a thicker structure which is undesirable because of increased optical absorption. Moreover, the active region--the bandgap of which determines the wavelength of the emitted light--is made with a semiconductor, such as GaAs or lattice-matched InGaAs, having a bandgap which is always less than those of the cladding regions and (any layers in) the mirrors, in order to avoid unwanted optical absorption in the cladding or the mirrors. The bandgap of AlAs is about 2.2 eV (corresponding to a vacuum wavelength of about 0.55 .mu.m) which is greater than the bandgap of about 1.4 eV (0.9 .mu.m) for GaAs. Consequently, a GaAs laser emits a wavelength of less than about 0.9 .mu.m.
In the cae of InP based lasers, longer wavelengths may be possible, namely, as long as 1.7 .mu.m. In such lasers, lattice-matching can be achieved, for example, with the ternary semiconductor Ga.sub.0.47 In.sub.0.53 As ("lattice-matched GaInAs") or with quaternary semiconductors, such as Ga.sub.x In.sub.1-x As.sub.y P.sub.1-y or Al.sub.x Ga.sub.y In.sub.1-x-y As. Again, in prior art the bandgap in the active region has been always selected to be less than those of the cladding regions and mirrors, to minimize unwanted optical absorption in the cladding and mirrors, and thereby minimize the threshold current for lasing (laser operation). Again also, in order to achieve reasonably large reflectivity at each interface between two contiguous layers in the mirror stack, the chemical compositions of the two layers have been selected to be, as they should be, reasonably disparate to achieve a reasonably large difference in their refractive indices and henced a reasonably large reflectivity at their interfaces. However, such a requirement in turn entails undesirably high optical absorption, whereby the threshold for lasing is undesirably high. For example, in an InP based laser in which the active region is composed of lattice-matched GaInAs, the cladding regions of InP, and the mirror(s) of alternating quarter wavelength layers of lattice-matched GaInAs and InP--optical absorption in the GaInAs mirror layers in the mirror stack is undesirably high, so that the threshold for lasing is also undesirably high. Although this absorption can be reduced by merely adding phosphorus to the lattice-matched GaInAs in the mirror layers and simultaneously changing the atomic ratio of Ga/In in the mirror layers to form lattice-matched Ga.sub.x In.sub.1-x As.sub.y P.sub.1-y --as is described, for example, in a paper entitled "GaInAsP Surface Emitting Laser (.lambda.=1.4 .mu.m, 77K) with Heteromultilayer Bragg Reflector," published in Electronics Letters, Vol. 21, No. 7, pp. 303-304 (28 Mar. 1985)--such a procedure would undesirably decrease the reflectivity at the interfaces between the (original) InP layers and the resulting Ga.sub.x In.sub.1-x P.sub.1-y, whereby (because of the resulting smaller difference in refractive indices) undesirably larger numbers of quarter-wavelength layers in the mirror stacks would be required to maintain the desired high total reflectivity of the stack. In turn, these larger numbers of layers in the mirror stack would undesirably increase the optical absorption of the laser structure per optical pass through the structure. Moreover, this absorption problem would be even more severe if in an effort to achieve a shorter wavelength optical output (larger energy per photon) the active region would be made of a multiple quantum well structure which would generate optical radiation of such shorter wavelength.
It would therefore be desirable to have an InP based laser structure which mitigates this problem.